Figure 2. The concentration-response curves of the effect of isoproterenol alone ([square, open]) and with the addition of 1.3 minimum alveolar concentration (MAC) halothane ([square bullet, filled]) or 1.3 MAC isoflurane ([round bullet, filled]) on the isometric force of contraction of rat papillary muscles. All values are given as means with 95% confidence intervals. Ordinate = the force of contraction in the percentage of predrug value; abscissa = the molar concentration of isoproterenol. Table 1shows the basal force of contraction and the increase in the force of contraction.